Information: Bolbelasmus unicornis was originally described as Scarabaeus unicornu by Schrank von Paula in 1789 and later as S. aeneas by Panzer in 1793. Since the late 18th century, the species has often been confused with Scarabaeus quadridens Fabricius, 1781, from India, and was later synonymized with it.
The type locality (locus typicus) of Bolbelasmus unicornis (originally described as Scarabaeus unicornus Schrank, 1789) is not explicitly stated in the original description, which is common for older taxonomic works. However, based on available evidence, the area around Vienna (Wien), or more broadly Lower Austria (Niederösterreich), is generally considered the most likely type locality. This assumption is based on the fact that Franz von Paula Schrank worked mainly in the Vienna and Bavaria regions, and most early specimens originate from this area.
Bolbelasmus unicornis is a European species of ground beetle that extends into the western Asian part of Turkey with a distribution center in the Pannonian Basin. It is a medium-sized beetle of the genus Bolbelasmus, which was the subject of considerable interest among insect collectors already in the 19th century due to its interesting and attractive appearance.
Due to its secretive lifestyle and lack of knowledge about effective collection methods, the distribution and bionomics of B. unicornis are poorly known. Adults spend most of their time underground, with above-ground activity limited to a short flight period when they move very close to the ground just after sunse. Nothing is known about the immature stages and the diet of adults and larvae. However, some authors have assumed that both adults and larvae feed on hypogeal fungi. Adults, like other members of the genus Bolbelasmus, are capable of loud stridulation.
Bolbelasmus unicornis possesses a strongly convex, globular body shape. The coloration is typically reddish-brown with a glossy surface; freshly emerged individuals may exhibit a bright orange hue. Males have a short, immobile horn on the frons and exhibit four small tubercles and two large depressions on the anterior pronotum. In contrast, females lack the frontal horn, and their pronotum is smooth, without any elevations or indentations. The scutellum is sparsely and barely visibly punctate.
Body length: 9.5 - 15 mm
Peak activity: May - September
Remarks: Bolbelasmus unicornis shows a strong preference for lowland habitats, particularly in warm, open landscapes such as floodplains, sandy river terraces, forest-steppe mosaics, and pasturelands. The species is often found in areas with deep, loose, and moist soils—especially sandy or loess substrates—which are essential for its burrowing behavior. Habitats typically include grazed meadows, open oak woodlands, orchards, and edges of forests. Crucially, the beetle is associated with traditional, extensively managed landscapes, and it avoids areas that are intensively farmed or heavily urbanized. Moisture availability in the soil, especially following heavy rains, appears to be a key factor influencing both habitat suitability and flight activity.
In Bolbelasmus unicornis, adults remain underground for most of their lives, with above-ground appearances restricted to brief flight intervals after sunset. Occasionally, individuals have also been seen crawling during daylight. The species tends to initiate flight around 35 minutes post-sunset on average, with observed start times ranging from 23 to 52 minutes, and flights typically ending by 60 minutes after sunset (extending between 35 and 86 minutes in total duration). These flight events usually occur when air temperatures average 21 °C, though the full range spans from 14 to 26 °C. Interestingly, related species have been documented flying in even colder conditions. Flights in B. unicornis only take place following heavy rainfall, once the soil is moist to a depth of approximately 30 cm. Wind conditions play a significant role: most activity is noted when the air is still, while strong winds seem to inhibit flight altogether. However, light rain or fog does not appear to interfere, and there are even records of flight during moderate rainfall. Typically, the beetles fly slowly, just 20–50 cm above the ground, often hovering in place. In windy environments, however, they tend to move faster and ascend to heights of 1–2 meters.
In the Pannonian Basin, which is the center of distribution of Bolbelasmus unicornis, adults are active from May to September, exceptionally from April and still in October, with a significant peak in June and the first half of July.
The species has not been recorded in France, (Slovenia), Bosnia and Herzegovina, Albania and Moldova for more than 50 years. It is probably extinct in France, Switzerland, Poland and the Czech Republic.
Distribution: Albania??, Austria, Belarus??, Bosnia and Herzegovina, Bulgaria, Croatia, Czech Republic??, France??, Germany, Greece??, Hungary, Italy, Macedonia??, Montenegro ??, Moldova, Poland??, Romania, Slovakia, Slovenia, Serbia, Switzerland ??, Turkey, Ukraine
Zoogeographic region: Palearctic
Taxonomic classification:
Bolbelasmus unicornis male & female
Material examined (& observation):
Our observation period: June ~ August 
Sampling Methods: Attracted to light trap 
Slovakia
The genus Bolbelasmus in the western and southern regions of the MedittBasin, Hillert et al 2016.pdf
Current and predicted distr of the rare and threatened beetle B unicornis in Serbia, Čurčič 2019.pdf
